OUTCROP-SCALE BRITTLE FAULTING IN THE CENTRAL VIRGINIA SEISMIC ZONE: PALEOZOIC TO MESOZOIC RELICTS, OR EVIDENCE FOR CENOZOIC NEOTECTONISM?

Brittle, mostly reverse faults exposed at a construction site in Louisa, Virginia ~15 km north of the epicenter of the 23 August 2011 M5.8 earthquake, share characteristics with many other documented brittle faults in the Central Virginia seismic zone (CVSZ). Faults, with ≤1 m offset, cut saprolite developed in Cambrian Chopawamsic Formation schist and intrusive felsite and pegmatite, on the SW margin of the Ellisville pluton. All faults are marked by cm-thick seams of tan, gray and red clay gouge, or are coated with mm-thick manganese oxide. Polished slickenlined surfaces are common in both clay gouge and on manganese oxide coated surfaces.

The longest faults, with ≤10 m fault traces, are east-verging reserve faults that trend ~190° to 210° and are convex upward. Northwest dips of 70° occur at the base of the excavation walls, but shallow to ~25° NW as the faults approach the B-soil horizon near the top of the ≥2 m high excavated cuts. Evidence for deformation in the soil profile was inconclusive. Subsidiary conjugate faults, ≤2 m in length, trend 085° to 095°, 160° to 175° and 215° to 240°, with dips from 25°-80°. Many of these apparent dip-slip and lateral-slip faults are truncated at, or branch from, the main faults. Few of the faults parallel ductile foliation or lithologic contacts.

Faults at the Louisa site appear identical to the many thin, clay-filled fault seams that offset features in bedrock saprolite in the Roundabout Farm trenches excavated shortly after the 2011 event; two of the latter faults shallow as they sole into the base of Quaternary residuum. At Battlefield Farms near Everona, Virginia, clay-filled and slickenlined brittle faults were exposed during construction in the early 1980s. A NE-trending, east-vergent, convex-upward fault (the Everona fault) rises through saprolite and displaces undated surficial deposits possibly as young as Pliocene.

Brittle faults at all of these sites superpose NW-contractional Paleozoic ductile faults. Outcrop-scale brittle fault overprints have been observed along ductile faults throughout the CVSZ. With the exception of the Everona fault, which is clearly Cenozoic, possible ages of these brittle faults are poorly constrained and could range from late Paleozoic or Mesozoic to as young as Quaternary.